Old fishermen once downed a cupful to keep warm when venturing out to sea. Divers drank it before plunging into deep, cold waters. Many believe the best kind comes from only one island, where it is aged in decades-old barrels of a particular type of wood.

Who knew fermented fish could be so romantic?

Like wine in France and olive oil in Italy, fish sauce is the prized staple of Vietnam, where it is used in soups and marinades or diluted into a sauce that accompanies foods from spring rolls to noodles. The Vietnamese have seals on their bottles to indicate quality, the highest being nuoc mam nhi, the first extraction of liquid from fish fermented in salt: extra-virgin fish sauce, if you will.

And the best of the best, as widely agreed among Vietnamese enclaves around the world, comes from Phu Quoc, a tropical island off the nation's southwest coast. In fact, the Phu Quoc name is so coveted and abused in the fish sauce industry that local producers have been working with the World Trade Organization to protect its appellation of origin.

Curious as to what made Phu Quoc fish sauce so legendary, I made my way to the fabled island this winter to taste it for myself.

Frankly, there isn't much romantic about the Khai Hoan fish sauce factory I visited there. About 10 feet from the factory entrance, the acrid odor hit me: a dense, stifling smell, almost like rank sweat. Forging onward, I made my way into a tall room with floating rafters, underneath which stood row after row of hulking wooden vats, each with a spigot on the bottom to drain the juices. The giant barrels were filled with the amber-colored liquid, some with a crusty orange film that had settled on top.

At the factory storefront, a worker handed me a bowl of fish sauce and a straw and motioned for me to sip. A few drops filled my mouth with a pungent, robust meatiness. The flavor was rich and complex, like pure essence of cured meat compacted and liquefied.

Making the sauce requires three parts fish to one part salt, a ratio common to most producers in Southeast Asia. Anchovies or other tiny fish usually are used; larger, more expensive fish such as mackerel or sardines can be substituted but result in a costlier, less profitable product.

After about a week, liquid begins seeping from the fish and is drained and circulated back into the vat every day for an entire year -- long enough for it to reach concentration, but not long enough for hydrosulfuric acid to appear, which would spoil the taste.

This first extraction is the highest quality, reserved for direct consumption in dips and sauces. Subsequent extractions are produced by running sea water through the vat, which results in a weaker, lower-grade product normally used for cooking.

It's a process that hails from ancient times and is not confined just to Asia. The Romans used a similarly fermented fish liquid they called garum, which appears in nearly 350 recipes in Apicius's classical Roman cookbook, "De Re Coquinaria." Pompeii later became famous for its production of the condiment, and even now, a fish sauce called colatura remains a specialty of Cetara, a village on the Amalfi Coast of Italy, where locals toss it with pasta and garlic. In the East, fish sauce is likely to have come from China or Vietnam as a way of preserving fish. Some speculate that the Chinese often mixed in soy beans as filler, and because more of the population in China lived away from the coast, soy beans became more common than fish, eventually leading to the soy sauce now associated with Chinese cuisine.

At the factory in Phu Quoc, the workers lined up the bottles of fish sauce by gradients of color, like tea steeped to varying degrees. The darkest-colored bottle was labeled "43°N/1L" and came from the first extraction of liquid. The others bore decreasingly lower numbers -- 40, 30, 20 and 15°N/L -- and came from subsequent extractions, after water had been added.